1. Field of the Invention
The present invention relates to magnetic resonance imaging (MRI), and in particular to image enhancement in MRI using motion corrupted data.
2. Description of the Related Art
Methods are known in the prior art for obtaining and reconstructing images from MRI scanning, with various advantages and disadvantages in image processing for specific diagnostic goals. For example, three-dimensional (3D) high-resolution cardiac MRI (CMR), such as coronary MRI and late gadolinium enhancement (LGE) imaging, as well as MR angiography (MRA), is acquired in a segmented fashion over multiple heartbeats, which necessitates compensation of respiratory and cardiac motions. The latter is typically suppressed by imaging during the patient-specific rest period of the cardiac cycle. For high-resolution 3D sequences, when the acquisition cannot be completed within a single breath-hold, techniques for respiratory motion compensation have been developed.
For example, the respiratory navigator (NAV) is a known technique or method, in which a navigator echo is acquired to measure the displacement of the right hemi-diaphragm (RHD) during scanning. This displacement measurement is then used to determine whether or not the acquired imaging data should be retained for image reconstruction. For example, some NAV methods utilize a two-dimensional (2D) pencil beam typically positioned on the dome of the right hemi-diaphragm, and have been used to track respiratory motion. Due to a linear dependency between the respiratory motion of the heart and that of the RHD, NAV can be used to indirectly monitor the motion of the heart. In prospective NAV gating, the k-space lines, obtained from the k-space of MRI data, and which are acquired immediately after the navigator signal, are used for image reconstruction only if the NAV signal is within a pre-defined gating window.
Otherwise, the corresponding k-space lines outside the pre-defined gating window are rejected for possibly being motion-corrupted due to respiratory and/or cardiac motion, and thus such k-space lines would reduce image quality if included and processed during image reconstruction. Subsequently, new k-space lines are re-acquired in the next cardiac cycle. For a 5 mm. gating window, this typically results in an acceptance efficiency of 30-70%, in which the rejected lines are discarded and not used in the image reconstruction.
Alternative techniques have been developed to improve the efficiency of respiratory motion compensation. Prospective motion correction has been utilized in coronary MRI to achieve scan efficiencies of 80-100%. Retrospective motion estimation has also been used in coronary MRI to correct for the motion of the rejected lines for 3D radial trajectories with projection-based self-gating, and for sequences using image-based navigators. Self-gating with radial trajectories has been employed in LGE imaging as well, but image-based navigators may not be directly applicable in this case due to the inversion pulse applied prior to imaging.
Another major challenge in high-resolution cardiac MRI is the limited signal-to-noise ratio (SNR). In coronary MRI, administration of vasodilators, imaging at higher magnetic field strengths, and the use of exogenous contrast agents have been investigated as ways of improving the SNR for reconstructing images. In LGE imaging, the limited SNR can be improved by imaging over alternate heartbeats, thereby allowing more signal re-growth, although this approach also doubles the scan time. For 2D LGE imaging with breath-hold acquisitions, motion-corrected averaging has been utilized. However, due to long acquisition times and inter-average motion, multiple averages for 3D imaging are rarely used.
An improved image reconstruction method with a higher SNR would be advantageous in MRI applications to cardiac and cardiovascular diagnoses.